1. Field of the Invention
This invention relates to spinnerettes and has particular reference to spinnerettes suitable for the spinning of cellulose filaments from a solution of cellulose in a solvent, particularly a tertiary amine N-oxide..
2. Description of the Related Art
McCorsley U.S. Pat. No. 4,416,698, the contents of which are incorporated herein by way of reference, describes a method of producing cellulose filaments by dissolving the cellulose in a suitable solvent such as a tertiary amine N-oxide.
One of the features of such a system is that the solution, commonly referred to as a dope, is both hot and, if it contains a significant quantity of cellulose, viscous, requiring the use of extrusion pressures in the range 15 bar to 200 bar. Such pressures are similar to those experienced in melt-spun polymer systems, such as polyester systems.
Having produced the solution of cellulose in the solvent the solution is extruded or spun through a suitable die assembly including an unspecified jet to produce filamentary material which is passed into water to regenerate the cellulose by leaching out the amine oxide solvent from the extruded filaments.
The production of artificially formed filaments of material by extruding or spinning a solution or liquid through a spinnerette to form the filaments is, of course, well known. Initially, relatively small numbers of individual filaments were prepared, which filaments were individually wound up for use as continuous filament material. This meant that the number of continuous filaments which needed to be produced was essentially dictated by the number of filaments which could be individually wound either before or after drying.
However, if fibre is produced as a tow or if fibre is produced as staple fibre then different criteria apply to the number of filaments which can be produced at any one time. A tow essentially comprises a bundle of essentially parallel filaments which are not handled individually. Staple fibre essentially comprises a mass of short strands of fibre. Staple fibre can be produced by the cutting of dry tow or it can be produced by forming a tow, cutting it whilst still wet, and drying the cut mass of staple fibre.
Because there is no need to handle individual filaments in the case of a tow product or a staple product, large number of strands or filaments can be produced simultaneously.
Thus in the case of spinnerettes for the production of tow or staple fibre, in comparison to spinnerettes used for the production of continuous filament material, it is economically essentially to use spinnerettes with a large number of spinning holes.
Initially, a spinnerette for the production of continuous filament might have had 20 to 100 holes, with productivity being increased by the use of higher spinning speeds. With spinnerettes used for the production of tow or staple the numbers of holes can grow into thousands or even tens of thousands. Productivity can thus be increased by the use of more holes as well as higher speeds. Initially such spinnerettes with large numbers of holes were produced in thick plates, as in polyester jets. However, it is expensive and time consuming to produce large numbers of holes in such thick plates. Thus attempts were made to use thinner plates by taking a dish of metal and forming the holes through the dish to produce a spinnerette in the form of a dished member with the holes arrayed in some suitable pattern in the lower portion of the dish. Such a dish member was then bolted into a jet for the production of the spun material.
Unfortunately, however, the production of jets is a very expensive and time consuming process. Each hole has to be pierced individually. Very often the holes are of a complex shape and are produced by a series of drilling, punching or machining operations, which have only recently been semi-automated.
With any production process there is a risk of defects and for a given percentage defect level, however low, the absolute number of defects per jet will increase as the number of holes in the jet increases. This can mean that there reaches a stage where it is not practical to increase the number of holes in a single jet face because of the chances that the final product will have too many defects to be useful without subsequent refurbishment.
One way round this problem is the adoption of the so-called cluster jet or thimble jet. In a cluster jet a large number of small thimbles are produced each with a relatively small number of holes--say 1 to 1500 holes. Such cluster jets have been widely used in the production of cellulose filaments by the viscose process. The cluster jets can be manufactured relatively cheaply and if a defect is found in one hole the particular thimble can be rejected without losing the work of producing many thousands of holes. The thimbles are then inserted into a holder in such a way that the pressure of the dope or spinning solution acting within the spinnerette tends to firmly force the spinnerettes into the cluster jet holder assembly.
Such jet assemblies of the single dished jet with a large number of holes or a cluster jet are widely used in the production of viscose cellulose. Viscose cellulose is produced by wet spinning. Examples of such jets are to be found in Ullman Encyclopaedia of Industrial Chemistry, 5th Edition, 1987, volume A10, page 554.
Ullman also refers to the use of rectangular spinnerettes in the spinning of polyolefin fibres.
The present invention is concerned with the production and structure of a spinnerette particularly suited for the production of cellulose fibres from a solution of cellulose in a solvent. Such jets are further particularly useful for the production of staple fibre of cellulose from a solution of cellulose in a solvent such as amine oxide.
The spinnerettes of the invention are particularly suitable for use in the jets for the production of cellulose for the manufacture of staple fibre as described in the co-pending application for our common assignee number 08/066,777 filed on May 24, 1993 reference "Jet Assembly", now U.S. Pat. No. 5,527,178.